The traditional solution to the problem of allowing both transmit and receive signals over a single optical fiber requires spatial separation of transmit and receive laser beams outside the optical fiber. This spatial separation is conventionally accomplished using a planar lightwave circuit and edge-emitting lasers and waveguide detectors.
An example of the planar lightwave circuit approach is described in an article by H. Okano et al. entitled "Passive Aligned Hybrid Integrated Optical Module Using Planar Lightwave Circuit Platform" from the LEOS Conference, Boston, Mass., Nov. 18-21, 1996, pp. 73-74. Such planar lightwave circuit approach would be simplified if the transmit and receive laser beams could remain collinear. For such an approach to be feasible, the transmit and receive signals must be at different wavelengths.
As an example of the collinear approach, in an article by J. C. Bouley entitled "InP-based Photonic Integrated Circuits for future Optical Access Networks" from the LEOS Conference, Boston, Mass., Nov. 18-21, 1996, pp. 286-287, an edge-emitting laser transmits at a wavelength of 1.3 microns and a waveguide photodetector receives at a wavelength of 1.55 microns. The small tolerance for aligning an edge-emitter to fiber makes it difficult to couple single-mode fiber to edge-emitting lasers. The edge-emitting laser is an expensive distributed feedback laser or an etched facet laser because the waveguide photodetector behind the laser prevents the laser from being configured with a simple cleaved facet. It is difficult to control the amount of leakage from the back of the 1.3 micron edge-emitter laser into the 1.55 micron photodetector. This introduces optical crosstalk. The edge-illumination geometry requires compositional variations in the lateral direction (in the plane of the wafer), which leads to difficult growth and processing. A package using edge-emitting laser technology can be bulky and expensive.